Generally, secondary batteries are referred to as rechargeable batteries because they can be charged and discharged repeatedly unlike a primary battery incapable of recharging, and widely used as a power source in electronic devices such as cellular phones, notebook computers and camcorders, or electric vehicles. Particularly, a lithium secondary battery has an operating voltage of 3.6 V or more, which is three times higher than those of Ni—Cd batteries or Ni—H batteries mainly used as the power source of electronic equipments and has excellent energy density characteristics per unit weight, and thus, the use of the lithium secondary battery is rapidly increasing.
Such a lithium secondary battery mainly uses a lithium-based oxide and a carbon material as a cathode active material and an anode active material, respectively. Also, the lithium secondary battery may be classified as a prismatic battery, a cylindrical battery or a pouch-shaped battery.
The lithium secondary battery comprises an electrode assembly having a cathode/separator/anode structure, in which a cathode, a separator and an anode are sequentially disposed, and a cladding for sealing and receiving the electrode assembly together with an electrolyte solution therein. In particular, prismatic or cylindrical secondary batteries comprise, as a cladding, a case having an open end and a cap assembly sealed and welded with the open end of the case.
The electrode assembly may be constructed in a jelly-roll type structure obtained by interposing a separator between a cathode and an anode, followed by winding, or a stacking type structure obtained by interposing separators between multiple cathodes and anodes having a predetermined size, followed by sequentially stacking. The jelly-roll type electrode assembly has a high energy density per unit weight and is easy to manufacture and, in particular, easy to put in a case, thereby being widely used in cylindrical or prismatic batteries. Meanwhile, the stacking type electrode assembly is mainly used in pouch-shaped batteries.
In the charging/discharging process of a secondary battery, such an electrode assembly is apt to be deformed by repeated expansion and shrinkage. In the case of the jelly-roll type electrode assembly, stress may be concentrated in the center of the battery, making the electrode penetrate a separator and be in contact with a central metallic pin, thereby causing a short circuit. The short circuit may result in the heating of the battery, from which an organic solvent is decomposed to generate gas and increase the inner pressure of the battery, thereby bursting the cladding thereof. Such a gas pressure increase in the battery may also be caused by a short circuit due to external impact.
Such a safety problem of batteries may be overcome by the provision of a positive temperature coefficient (PTC) device (a safety device). In particular, a cylindrical battery comprises safety devices such as a current interrupt device (CID) for interrupting a current when the inner pressure of the battery increases, and a cap assembly which includes a top-cap constituting a protruded terminal for protecting the safety devices, and the cap assembly is sealed by a gasket to be fitted with a case.
However, while assembling the cap assembly with the case by the gasket, a gap may be formed between the gasket and the cap assembly or between the gasket and the case. By the formation of such a gap, there has often been a problem that the sealability of a conventional secondary battery deteriorates. That is, the gasket is generally pressed and deformed by the case by way of a clamping process, thereby making a close contact between the case and the cap assembly. However, since the surface of a conventional gasket that comes in contact with the case and the cap assembly therebetween is a simple flat surface, the contact becomes poor. Particularly, if the gasket is ununiformly pressed during the clamping of the case, the flat surfaces of the gasket is unevenly deformed and a part of the gasket does not come in contact with the case or cap assembly to form a space therebetween, thereby deteriorating the sealability of the gasket with the case. Accordingly, there has been a demand to develop a sealing structure between a cap assembly and a gasket.
Also, if a cylindrical battery having a cross section larger than the thickness of its case is exposed under a high temperature condition, a low-boiling-point solvent in an electrolyte solution may vaporize to increase the inner pressure of the battery and expand the battery case. Furthermore, the characteristics of the battery may be deteriorated by the leakage of the electrolyte solution and the increase of inner resistance due to the inlet of external air and humidity.
In order to overcome these problems, there is a need to improve the heat resistance of a gasket and an organic electronic material such as a sealant.